Organic solvent soluble and water-soluble or water-thinnable addition interpolymers containing pendent carboxyl groups, pendent amine groups formed by reacting a portion of the pendent carboxyl groups with an alkylenimine, and other pendent organic groups are known in the art as illustrated by U.S. Pat. Nos. 3,290,416; 3,290,417; 3,494,847; 3,719,692 and 3,945,963. While amine group-containing addition interpolymers of the type described in these patents have a number of advantageous properties, they also exhibit serious disadvantages which materially limit their usefulness.
A major disadvantage of amine group-containing addition interpolymers of the type described in these patents is their very limited storage life particularly at elevated temperature storage conditions. Thus, we have found that amine group-containing addition interpolymers of the type described in the patents are prone to gellation when stored at room temperature for time periods of less than one year (often much less) and when stored at elevated temperature (e.g., 200.degree. F.) for time periods of from about 16 hours to about 72 hours (often from 16 to 24 hours).
The addition copolymers of U.S. Pat. No. 3,719,629, while more stable than the others, exhibit other disadvantages. The water-thinnable addition copolymers of this patent are cationic acid salts, which creates a number of problems. First, the cationic nature of the copolymer means that it cannot be readily formulated into a coating composition by blending it with the conventional anionic modifiers normally employed in coating composition. Thus, acid sensitive pigments such as the carbonates and conventional anionic pigment dispersants cannot be readily utilized. Hence, the copolymers of the patent would ordinarily require the use of non-ionic modifiers which are not readily available commercially. Secondly, the fact that the addition copolymer product is an acid salt often precludes its use as a coating material for unprimed metal substrates since such acid salts usually cause flash rusting.
We have surprisingly and unexpectedly discovered that the storage stability problems encountered in addition interpolymers containing pendent carboxyl groups, pendent amine groups and other pendent organic groups (i.e., groups other than the carboxyl and amine groups) are related to the specific nature of the other organic groups included in the interpolymer. Thus, addition interpolymers containing pendent carboxyl groups, pendent amine groups and pendent organic groups formed from conventionally utilized ethylenically unsaturated monomers such as ethyl acrylate and methyl methacrylate have been found to be subject to gellation particularly at elevated temperatures (i.e., 200.degree. F.) whereas other addition interpolymers, similar except that the other pendent organic groups are formed from certain ethylenically unsaturated organic monomers such as isobutyl methacrylate, styrene and certain others (described in detail below), are not subject to gellation under the same conditions.
It has been further found that the storage stability problems involved in such amine group-containing interpolymers are at least to a major extent caused by internal aminolysis reactions. Aminolysis occurs as the result of a reaction between a primary or secondary amine group and an ester group thereby resulting in the formation of an amide. In the case of amine group-containing addition interpolymers of the type described in the above-mentioned prior art patents, it is believed that the aminolysis reaction occurs between the pendent amine groups formed by imination of pendent carboxyl groups and pendent ester groups formed from, for example, the esters of acrylic and methacrylic acid referred to as "soft monomers" in U.S. Pat. No. 3,929,743. The aminolysis reaction in such interpolymers is believed to proceed by a mechanism in which the pendent amine groups of one polymer chain react with the pendent ester groups of another polymer chain resulting in the formation of an amide and causing crosslinking between the polymer chains. This, in turn, leads to an increase in molecular weight of the copolymer and eventual gellation.
We have now found that the storage stability of amine group-containing interpolymers can be greatly improved by incorporating certain aminolysis-resistant organic monomers (described below) into such interpolymers.